T-SQL90 vs Transact-Sql

In Sql2012 Management Studio, there are separate options for "T-SQL90" and "Transact-Sql". My .sql files always seem to open in "Transact-Sql". My question is what is a "T-SQL90" file, how does it differ from a regular transact sql file, ...and how does Management Studio automatically decide what type of file it is? (different extension? if so, what is it?)